Assembly and Method for Performing In-Situ Endpoint Detection When Backside Milling Silicon Based Devices

ABSTRACT

An assembly for monitoring a semiconductor device under test comprising a mill configured to mill the device, a sensor configured to measure an electrical characteristic of the device, and a computer configured to determine the amount of strain in the device from the electrical characteristic when the mill is milling the device and detect an endpoint of milling at a circuit within the device. In use the endpoints of the milling process of the semiconductor device are detected measuring an electrical characteristic of the device with a sensor during milling determining the amount of strain in the device from the electrical characteristic and detecting an endpoint of the milling process within the device based on the amount of strain.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit to U.S. Provisional Application No.62/851,777, titled “Assembly and Method for Performing In-Situ EndpointDetection When Backside Milling Silicon Based Devices” and filed May 23,2019. The entire content of this application is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to failure analysis of a semiconductor,field effect transistor based device. More specifically, the inventionis directed to automatically determining the endpoint of a millingprocess without having to remove the silicon based device from a millingmachine during a failure analysis.

BACKGROUND

In semiconductor manufacturing, integrated circuit devices are typicallybuilt on a substrate. The substrate is preferably made of silicon.Logical circuit components are typically formed within a semiconductordevice and are connected through conductive paths in the substrate in anintegrated manner thus forming a semiconductor device commonly called anintegrated circuit. These components and conductive paths extenddifferent depths into the substrate and the entire assembly isencapsulated in resin or ceramic which serves to protect the device. Thesubstrate varies in thickness depending on processing and from device todevice. Substrates formed with different processes can vary by 10s to100s of microns while the variation in thickness from device to deviceis usually on the order of 10s to 100s of nanometers. Manufacturers ofsemiconductor devices will conduct a failure analysis on thesemiconductor devices to determine where faults in the devices arelocated so the devices can be redesigned to avoid the faults. Insemiconductor failure analysis, a large amount of the bulk silicon isremoved from the device on which a circuit is fabricated to supportadvanced failure analysis methods, such as optical imaging and opticalprobing. Such methods simply will not work without removal of enough ofthe silicon. In addition, Focused Ion Beam and Scanning ElectronMicroscopy both require some amount of backside thinning before use. Theremoval process is often conducted on a mill. In each case the millingprocess must be stopped at a so-called “endpoint” when enough siliconhas been removed to reveal an underlying circuit. The endpoint is simplya pre-defined “stop point” determined based on a combination of thesurvivability of the circuit and the mill being used. This endpoint ofthe milling process must be determined to avoid the mill damaging theunderlying circuit. Therefore, during removal of the bulk silicon, themilling process must be periodically paused and the device must beremoved so that the remaining silicon thickness can be measured todetermine if more silicon should be removed or the milling should bestopped.

In optical imaging, as with other analysis techniques, silicon must beremoved from the semiconductor device for the imaging to functionproperly, since the silicon interferes with the imaging. In opticalimaging, an interrogating beam is used to measure characteristics of thesilicon device in an area of interest within the silicon device. Awavelength of the interrogating beam plays two critical roles in theseadvanced interrogation systems. First, the wavelength defines theminimum size of an addressable feature that can be detected. In thiscase the resolvable feature is the smallest portion (also referred to asa “node”) of the circuit embedded in the silicon substrate that is beingstudied. This size is generally governed by the diffraction limit whichstates that there is a minimum resolvable feature size “D” to anyoptical system defined as:

D=λ/2·NA

where lambda is the wavelength of the interrogating beam and “NA” is thenumerical aperture of the lens. Second, the wavelength sets the depththe beam can penetrate the silicon. The depth is the distance from themilled surface of the silicon substrate to the circuit in thesemiconductor device. The material transparency or absorption of siliconchanges based on the wavelength of the light in the interrogating beam.Silicon has an absorption spectrum that is well characterized forwavelengths from the UV through IR spectrums. The absorption depth ofvarying wavelengths of light can be seen in FIG. 8. At 1319 nm, siliconis almost transparent whereas in the ultraviolet range of wavelengths,10 nm to 400 nm, silicon is almost completely opaque. Known opticalimaging techniques increasingly employ light having a wavelength nearthe ultraviolet range to resolve smaller features. These two conflictingfactors cause a tradeoff between absorption and feature resolvability.Advanced lens systems (solid immersion lenses, etc.) have been developedto increase the numerical aperture as high as 3.3 (unitless), decreasingthe diffraction limit. There are physical limitations (geometry andmaterials) to further increasing the numerical aperture. Therefore, thenumerical aperture in lenses is not expected to increase appreciably inthe foreseeable future. Due to these factors, at 1319 nm, even usingthese high numerical aperture lenses results in a resolvable featuresize significantly larger than circuit features in the most recentlydeveloped devices in the most advanced node size devices (sub-100-nmnode size). Therefore, it is desirable to remove as much silicon aspossible from a semiconductor device when under test, without destroyingthe circuits being analyzed.

The problem of absorption is also an issue when using energetic imagingtechniques such as electron microscopy and scanning electron microscopy.Silicon only becomes transparent to electrons when there is under(roughly) 5 microns of remaining backside silicon thickness between themilled surface and the circuit being analyzed. Even at this thickness,the transmission of secondary electrons through the silicon drasticallyreduces the ability of a scanning electron microscope to perform highresolution through silicon imaging and, once again, a small backsidesilicon thickness is required.

As can be seen from the above discussion, the backside silicon thicknessis a fundamental barrier in optical probing. By either removing orreducing the backside silicon thickness, many optical probing techniquesregain their viability in integrated circuit failure analysis. Therequired thickness removal varies by the wavelength desired and thepower of the interrogating optical system. However, for many of thesetechniques it is essential that the device remain operable and thecircuit be unaltered. To address the problem of too much bulk backsidesilicon on integrated circuits, many commercial solutions have beendeveloped to remove the backside silicon through mechanical or chemicalmilling. Each technique has a trade-off when compared to the other.Mechanical milling is typically less selective and more difficult tocontrol but is far more rapid. Chemical processing is slow, non-uniform,and can sometimes unintentionally affect other systems of the devicesuch as occluding the silicon, damaging an interposer between anintegrated circuit die and the metal contacts or corroding the metalcontacts themselves. Because of these factors the industry tends tofavor mechanical milling over chemical removal of silicon.

There are several products on the market designed to mechanically removebackside silicon. The ASAP-1, that is produced in various forms byUltraTec, is a budget backside mill which is capable of gross removal ofmaterial. The ASAP-1 is an exclusive end mill with 3 axes (x, y, zmilling) which means that it is only capable of applying downward,relatively constant force. The X-Mill, produced by Allied Hightech Inc.,is a combination end and edge mill. The X-Mill uses both force feedbackand dead reckoning methods to determine endpoints for the millingprocess. These methods are done in a discrete step fashion by measuringthe device thickness on a measurement system, moving the milling head,milling the backside silicon, moving the device to the measurementsystem, and measuring again. This method provides better accuracy byproviding an updated starting point before each milling cycle. However,the remaining silicon thickness does not indicate if the device is stillfunctioning. Also, all of these tools, at some stage in their workflow,require a normal force to be applied to the backside silicon which cancompromise device integrity. A simple model of the load/strainrelationship is shown in FIG. 9 showing how strain can suddenly changeresulting in damage to the device.

What all these devices lack is an in-situ endpoint detection system.Each tool has an algorithm for attempting to mill as close as possibleto the milling endpoint desired. However, for each new semiconductordevice there is a process of adjusting the mill to the new semiconductordevice, using surrogate stand-in devices, to reduce risk of destroyingthe new device. Devices are regularly destroyed during this processeither due to overmilling (i.e. milling through the circuit) or due tothe normal force applied to the thinned backside silicon. Even afterrisk reduction is performed, backside milling techniques represent ahigh risk to the sample device of interest. Because of this risk,enormous efforts are made to keep backside milling out of the failureanalysis process until all other options have been exhausted, even insituations in which backside techniques are likely to be the mosteffective. After the decision to use backside milling is made, valuabledata is still at high risk for loss due to device destruction.Therefore, there exists a need in the art for a method for performingin-situ endpoint detection when backside milling silicon basedsemiconductor devices.

SUMMARY

As noted above, the ever decreasing size of the smallest feature of anintegrated circuit is requiring advanced methods of failure analysiswhich require the removal of backside silicon from the integratedcircuits being tested to enable imaging and interrogation of the fieldeffect transistor layers of the integrated circuit. To reduce risksassociated with backside milling of a semiconductor device, the presentinvention provides an in-situ device function monitoring tool and methodfor detecting the endpoint of a milling operation. The method uses thepiezoelectric mechanism of field effect transistors to actively monitorthe power draw of an integrated circuit as the backside silicon ismilled. When the downward pressure of the micromill applies strain tothe silicon substrate, the electron mobility of the field effecttransistors is changed. This property results in a detectable change inthe power draw of the device preceding permanent device deformation andassociated damage. In one embodiment, an automated feedback system isembodied in a controller for a micromilling machine is used to detectchanges associated with strain in the field effect transistors in thesemiconductor device being milled and reduce or stop the force ofmilling when the endpoint of milling is reached before the semiconductoris destroyed.

An assembly is used to monitor the in-situ strain signals through poweranalysis while a device under test, such as an integrated circuit in asilicon based semiconductor device, is undergoing backside milling. Theassembly monitors a semiconductor device under test, including a millconfigured to mill the device, a sensor configured to measure anelectrical characteristic of the device, and a computer configured todetermine the amount of strain in the device from the electricalcharacteristic when the mill is milling the device and detects whetheror not the device is still functioning. The computer can characterizethe thickness of the backside silicon and then indirectly determine thethickness of the remaining backside silicon layer within the device. Themill is preferably a micromill including a support for the semiconductordevice that moves in both X and Y directions of a milling surface, afeed motor which moves the support in a Z direction normal to themilling surface and a sensor for determining an amount of force appliedto the device. The computer is further configured to control the millwhen the device is undergoing milling. Preferably, the micromill isconfigured to support a socket holding the device. The socketimmobilizes the device with respect to the micromill and allows accessto a backside or frontside of the device. The micromill employs millingfluid and the socket prevents contamination of the electrical contactsof the device by the milling fluid. The assembly also has a signalgenerator configured to send a signal to the device and produce themeasured electrical characteristic. The signal generator and anoscilloscope are connected to the socket when the socket is placed inthe micromill or to lead wires connected to the device. The signalgenerator is preferably configured to apply a clock or a power waveformto the device. Alternatively, the device is soldered to a printedcircuit board which is coated with a material providing chemical andelectrical passivation. The sensor may be the oscilloscope or,alternatively, is a current or a voltage sensor connected to a digitalto analog converter. The sensor could also be an RF sensor.

In use, the assembly mills the semiconductor device in the mill whilemeasuring an electrical characteristic of the device with a sensorduring milling and both determines the amount of strain in the devicefrom the electrical characteristic, and detects an endpoint when thefunctionality of the device is reversibly altered. Preferably thicknesscan be characterized in multiple runs but this is not the primaryfunction of the device. This results in a method of in-situ monitoringof the power draw of an integrated circuit which is correlated to thestrain, under load, of the integrated circuit.

The preceding summary is provided to facilitate an understanding of someof the innovative features unique to the present disclosure and is notintended to be a full description. A full appreciation of the disclosurecan be gained by taking the entire specification, claims, drawings, andabstract as a whole.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing description of various illustrative embodiments in connectionwith the accompanying drawings.

FIG. 1A shows a schematic view of an overall assembly for millingsemiconductor devices in accordance with a preferred embodiment of theinvention. The assembly is shown with a signal generator, a micromillwith a socket, an oscilloscope and a data analysis system with a triggerconnection to the micromill.

FIG. 1B shows a schematic view of an assembly similar to the assembly ofFIG. 1A wherein the oscilloscope has been replaced with a sensor inaccordance with another preferred embodiment of the invention.

FIG. 1C shows a schematic view of an assembly similar to the assembly ofFIG. 1A wherein the signal generator, the oscilloscope and the dataanalysis system have been combined into one unit.

FIG. 2 shows a flow of information between several components of thedata analytics device.

FIG. 3 shows a micromill holding a semiconductor device.

FIG. 4 shows a cross section of a semiconductor device.

FIGS. 5A, 5B and 5C show a semiconductor device responding to differentlevels of applied force.

FIG. 6 is a flow chart of a milling process.

FIG. 7 is a graph showing how a semiconductor responds to differentamounts of pressure.

FIG. 8 is a prior art graph showing light absorption depths of siliconat various wavelengths of light.

FIG. 9 is a prior art model of a load versus strain at variousthicknesses of a semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description should be read with reference to thedrawings in which similar elements in different drawings are numberedthe same. The detailed description and the drawings, which are notnecessarily to scale, depict illustrative embodiments and are notintended to limit the scope of the disclosure. The illustrativeembodiments depicted are intended only as exemplary. Selected featuresof any illustrative embodiment may be incorporated into an additionalembodiment unless clearly stated to the contrary. While the disclosureis amenable to various modifications and alternative forms, specificsthereof have been shown by way of example in the drawings and will bedescribed in detail. It should be understood, however, that theintention is not to limit aspects of the disclosure to the particularillustrative embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the disclosure.

As used in this specification and the appended claims, the singularforms “a”, “an” and “the” include plural referents unless the contentclearly dictates otherwise. As used in this specification and theappended claims, the term “or” is generally employed in its senseincluding “and/or” unless the content clearly dictates otherwise.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

As used throughout, any ranges disclosed herein are used as shorthandfor describing each and every value that is within the range. Any valuewithin the range can be selected as the terminus of the range.

As best seen in FIG. 1A, an assembly 100 is comprised of a signalgenerator 110, a socket 120 configured to receive a semiconductor device125, which may include an integrated circuit, a mill or micromill 130,an electrical signal recording device such as an oscilloscope 140 and adata analysis computer 150. Preferably micromill 130 includes apressure-based grinding system. Semiconductor device 125 is connected tooscilloscope 140 and signal generator 110 via socket 120. Morespecifically, a first communication line 160 is provided from signalgenerator 110 to socket 120 and a second communication line 170 isprovided from oscilloscope 140 to socket 120. Oscilloscope 140 is alsoconnected to data analysis computer 150 through a third communicationline 180. Data analysis computer 150 can interface via a trigger,represented by line 190 with micromill 130.

Socket 120 may be any socket which allows for temporary placement andelectrical connection to signal generator 110 and oscilloscope 140.Preferably, socket 120 immobilizes semiconductor device 125 with respectto micromill 130 and prevents contamination of electrical contacts bymilling fluid. Socket 120 may be semi-permanent via solder or conductiveattachment. Socket 120 may allow back side access or front side accessto semiconductor device 125. In some embodiments semiconductor device125 is soldered or otherwise permanently wired to lead wires representedby communication line 170 which are connected to oscilloscope 140 andsignal generator 110. In these cases, socket 120 is not needed. In someembodiments, socket 120 will be replaced by a printed circuit board (notshown) to which semiconductor device 125 is soldered. Preferably, theprinted circuit board is coated with a material after soldering toprovide chemical and electrical passivation. Parylene, PDMS, or anothercommon encapsulates are used for this purpose.

Signal generator 110 may be any clock-generating device that can be usedto apply a clock or power waveform 200 to semiconductor device 125either directly or through a specified socket 120. Signal generator 110could be as simple as a single board microcontroller (i.e. Arduino),FPGA, development board, single board computer, or complex as aprofessional-grade signal generator. One or more signals 200 are appliedto semiconductor device 125. In a most preferred embodiment, a singlesignal is sent to a clock of semiconductor device 125 causing power drawsignal 210 which is measured by oscilloscope 140.

Oscilloscope 140 may be replaced with any electrical signal recordingdevice such as a side-channel or secondary-effects measurement device.As milling occurs, semiconductor device 125 will continue to produceundisturbed primary effects; indeed, if primary effects are disturbed,milling has caused device failure and sample semiconductor device 125 isno longer usable. As shown in FIG. 1B, oscilloscope 140 has beenreplaced by a box 240 representing a number of secondary effectmeasurement devices which may be used instead of oscilloscope 140. Forexample, a current sensor whose output is digitized via a high speedanalog to digital converter could be used. Alternatively, a voltagesensor could be used in place of a current sensor. A thermal sensorcould also be employed. Finally, an RF antenna and signal chain could beused to look for variations in RF energy related to strain on the die insemiconductor device 125. Data analysis computer 150 may be any PC orminiature computer; standalone signal analyzer; or signal processingunit built-in to the oscilloscope or any other signal measurementdevice.

In some embodiments, as represented by box 350 shown in FIG. 1C, thefunctions provided by signal generator 110, oscilloscope 140, and dataanalysis computer 150 may be implemented in the same controller 350.Controller 350 may take many forms including a field-programmable gatearray, a microprocessor development board including a printed circuitboard containing a microprocessor and minimal support logic, a personalcomputer, or a controller integrated within the micromill electronics.Further details of these components are found in U.S. Pat. No.10,054,624, incorporated herein by reference. Regardless of whether thecomputer is one separate unit or combined with other units, the overallarrangement will preferably follow the logic shown in FIG. 2 Namely asensor 410 measures power draw from semiconductor device 125. An analogconditioner 420 receives signal 200 as an analog signal power versustime. Digitizer 430 and digital conditioner 440 segments signal 200 toproduce a digital vector representing the signal which is then processedby various machine learning algorithms 450 to determine the second ordereffects on the power draw and how the second order effects relate tostrain in semiconductor device 125. Algorithms 450 can then determinewhen to send trigger signal 180 to micromill 130 to prevent damage tosemiconductor device 125. Dependent on the desired impact, trigger 190will act differently on micromill 130. In some configurations, trigger190 will increase or reduce applied pressure, increase or decreasemilling bit rotational speed, increase or decrease feed velocity in thehorizontal plane, increase or decrease feed velocity in the verticaldimension, increase or decrease stage angle, increase or decreasespindle/collet/milling bit angle, start or stop milling, turn on and offthe mill or any combination of the above specified actions.

FIG. 3 is a schematic representation of the micromill 130 which includesa tilt table 514 for supporting and oscillating supported semiconductordevice 125 in the X and Y directions. Two drive motors are disposed in abase element 520 serve to oscillate table 514 as is well known in theart. Both the speed and the amplitude of the oscillations in the X and Ydirections are independently adjustable via controller 522. A tool 524is rotated at an adjustable speed by an element 528 which is movable inthe Z direction. The speed of rotation is adjustable via input fromcontroller 522. A precision Z motor 530 controls the feed velocitynormal to the milling surface. Micromill 130 uses a pressure sensor 540to determine force which is applying to spinning tool 524. This methodof contact is known as end milling, as the end of the milling tool 524removes the material. This pressure ranges from 0-1000 g of force. Asimilar mill is described in U.S. Pat. No. 6,620,369, incorporatedherein by reference. Alternative embodiments of the micromill includeadditional milling axes, an edge mill (as opposed to an end mill), amill employing Laser Milling, Laser Assisted Chemical Etching, orFocused or broad ion beam milling.

FIG. 4 shows a cross section of semiconductor device 125 with a portionof epoxy layer 610 completely removed. Also, a portion of silicon layer620 is removed leaving a remaining thickness 630 between a millingsurface 640 and a die or circuit 650. Circuit 650 is connected to apower contact or pin 660 and a ground contact or pin 670 by conductors680 and 690 respectively.

Assembly 100 employs the inherent nature of the semiconductor materialsto detect damaging strain in a semiconductor device 700 that includes anintegrated circuit 705. As shown in FIGS. 5A-5C there is shown circuitof a semiconductor device 700 under the application of a force shown byan arrow 710. In FIG. 5A no force is applied and an input signal tosemiconductor device 700 is faithfully produced as an output signal 720.In FIG. 5B as force 710 is applied the behavior of circuit 705 begins tochange. If the pressure is reduced in response to increasing strainsemiconductor device 700 will return to normal operation shown in FIG.5A. If the pressure is not reduced in response to increasing strain,thinning of the silicon by milling will result in device 700 breaking asseen in FIG. 5C. The large deformation cause collisions between metallayers in circuit 705, resulting in shorts and therefore large increasesin current draw.

As shown in FIG. 6, a method 800 of in-situ monitoring of a principlecomponent analysis of the power draw of semiconductor device 125 isemployed to detect a change in strain. Method 800 is employed to removebackside silicon layer 620 in FIG. 4 from semiconductor device 125 toexposed integrated circuit 650, thus allowing circuit 650 to be analyzedfor faults using the optical techniques described above.

Initially, at step 810, semiconductor device 125 mounted on table ofmicromill 130, as shown in FIG. 3. Semiconductor device 125 may be, forexample a PIC16 microcontroller manufactured by the Microchip companyhaving at least a power pin connection 660 and a ground pin connection670. Next, signal generator 110 and oscilloscope 140 are connected tosemiconductor device 125. Preferably, signal generator 110 is connectedto power pin connection 660 and oscilloscope 140 is connected to groundpin 670. Next micromill 130 is used to remove protective epoxy layer 610on semiconductor device 125.

At step 820, micromill 130 is controlled to mill backside silicon layer620 of semiconductor 125 at the same time, signal generator 110 isadjusted to send a signal to semiconductor device 125. Preferably, ablock signal 200 is sent but various different types periodic signal maybe employed. Oscilloscope 140 measures a response signal fromsemiconductor device 125, caused by block signal 200 at ground pin 670.While silicon layer 620 of semiconductor device is being milled, theresponse signal is continuously measured. Also, a force is applied tosemiconductor device 125. As micromill 130 mills through silicon layer620 and gets closer to the actual circuit or die 650 withinsemiconductor device 125, the strain on die 650 increases and causes achange in power draw signal 210. The force can come from the millingperformed by micromill 130 or be generated internally withinsemiconductor device 125. The force within die 650 is also caused by thedie's heterogeneous structure having two disparate materials bondedtogether which create stress when cyclically loaded and that in turncreates strain. As the milling process removes more and more silicon,semiconductor device 125 becomes more flexible allowing it to strainmore. While not wishing to be bound by theory, it is believed that thechange in power draw signal 210 is caused by the change in carrierdensity in the actual semiconductor junctions that change as a functionof strain. The power draw is correlated to the strain, under load, ofthe integrated circuit. Assembly 100 is used to monitor the in-situstrain signals through power analysis while semiconductor device 125 isundergoing backside milling. A typical integrated circuit 650 insemiconductor device 125 contains many field effect transistors. It istheorized that this measurable change in the power draw, or moreprecisely the measurable second order effect on the power draw is due tothe change in electron mobility of the field effect transistors whenstrained. Regardless, once micromill 130 reaches the endpoint of themilling process when micromill 130 has removed enough of the siliconcovering the die, the milling is stopped. Typically the mill is stoppedbefore the silicon is completely removed otherwise the device will notfunction.

Next at step 830 a power analysis is performed using machine learningtechniques to determine the normal power signature of semiconductordevice 125 under minimal strain. Preferably, power draw signal 210 ismeasured over time when semiconductor device 125 subject to clock signal200, although numerous other values such as voltage over time or currentover time could be measured. Power draw signal 210 may be segmented toconvert a measured analog power signal into a set of discrete valuesthat represent the power signal. The segmented signal may be referred toas a feature vector. The feature vector can be transformed into thefrequency or a different time independent domain. In one embodiment,each feature vector is transformed with a discrete fourier transform orfast fourier transform. In alternative embodiments, each feature vectormay be transformed with a discrete cosine transform, Hilbert transform,real cepstrum, wavelet coefficients, or a hybrid of several differenttransforms. The dimension of the feature vector can be reduced usingessentially any known dimension reduction technique. Preferably,principal component analysis is conducted to reduce dimensionality onthe feature vector. Principal component analysis transforms the featurevectors into a space where the greatest variance between samples is inthe first dimension, the next greatest variance in the next dimensionand so on. By organizing the feature vectors by greatest variance,dimensions where the least variance between samples occurs can bediscarded in order to enable comparisons in a lower dimensional spacewith conventional distance metrics. Although the current embodimentimplements principal component analysis, other non-linear analysistechniques may be employed instead such as self organizing maps or othermanifold based learning algorithms. In one embodiment, principalcomponent analysis on the feature vector to reduce dimensionality of thefeature vector includes organizing the feature vector by variance anddiscarding dimensions where the variance is below a threshold. Inanother embodiment, principal component analysis on the feature vectorto reduce dimensionality of the feature vector includes organizing thefeature vector by variance and discarding all but a predefined number ofdimensions that have the highest variance. In addition, a clusteringanalysis may be conducted of the vectors to increase accuracy ofdetermining the strain in the semiconductor device. Further details ofthese machine learning techniques are found in U.S. Pat. No. 10,054,624and U.S. Patent Application Publication No. 2018/0307654, bothincorporated herein by reference.

At step 840 the device is monitored while under load. Strain is afunction of the cross-sectional profile of homogeneous material. As thethickness goes to zero the strain increases as t³, where t is thethickness and all other factors being equal. By monitoring the changefor a sharp increase in the electrical characteristics as a function ofstrain, the endpoint is detected and the load on the semiconductordevice can be reduced when the milling endpoint is detected to avoidirreversible damage. The triggers sent from the computer can thereby becontrolled to stop milling when the milling endpoint has been detected.This technique has been demonstrated using a PIC16 microcontroller. Inthis demonstration, shown in FIG. 7, the device was damaged do tooverloading (800 mg) but this is significantly higher than standardmilling profiles. As shown in FIG. 7 the strain is reversible afterloading. Therefore, the strain can be detected at lower loading valuesand the milling stopped before irreversible strain is reached.

Having thus described several illustrative embodiments of the presentdisclosure, those of skill in the art will readily appreciate that yetother embodiments may be made and used within the scope of the claimshereto attached. Numerous advantages of the disclosure covered by thisdocument have been set forth in the foregoing description. For examplewith the disclosed method a single integrated device may be tested bybackside milling method without risking destruction of the device. Thiscontrasts with the prior art method which tended to destroy severalidentical devices before successfully measuring one of the devices. Itwill be understood, however, that this disclosure is, in many respects,only illustrative. Changes may be made in details, for example themethod looks for significant changes in trend to indicate device stress.This trend identification can be performed through multiple methods. Thepresent embodiment uses principle component analysis on a currentwaveform. Changes of the principal components over time signify thestress of the sample. Thresholding, rate of change, and other similaralgorithms may also be used to indicate critical stress, and thus thestoppage condition. The same algorithm can be applied to other types ofsensors. Other algorithm types can include any algorithm that producestrend information from a sample, such as lossy compression, neuralnetworks, curve fitting, and machine learning. The data can bepreprocessed in the frequency domain, e.g., fourier transforms, andwavelets, or the time domain, e.g., down sampling, and smoothingfilters. Because the nature of internal stresses in modern integratedcircuits this method may also be used to determine alterations to acircuit undergoing non-contact-based milling. This may include a focusedion beam, chemical removal, or a laser-based system which does not exertdirect force on the backside of the device. The disclosure's scope is,of course, defined in the language in which the appended claims areexpressed.

What is claimed is:
 1. An assembly for monitoring a semiconductor deviceduring milling comprising: a sensor configured to measure an electricalcharacteristic of the semiconductor device during milling in a mill, anda computer configured to determine an amount of strain in thesemiconductor device from the electrical characteristic when the mill ismilling the semiconductor device and to detect when the milling shouldstop before a circuit within the semiconductor device is damaged.
 2. Theassembly of claim 1, wherein the computer is further configured toregulate the mill when the semiconductor device is undergoing millingand determine an amount of silicon left between the milling machine anda circuit located in the semiconductor device based on the electricalcharacteristic.
 3. The assembly of claim 1 further comprising a socketconfigured to receive the semiconductor device.
 4. The assembly of claim3, wherein the mill is a micromill configured to hold the socket.
 5. Theassembly of claim 4, wherein the socket immobilizes the semiconductordevice with respect to the micromill and allows access to a backside ofthe semiconductor device and the semiconductor device is an integratedcircuit.
 6. The assembly of claim 4, wherein the micromill employsmilling fluid, the semiconductor device includes electrical contacts andthe socket prevents contamination of the electrical contacts by themilling fluid.
 7. The assembly of claim 4 further comprising a signalgenerator configured to send a signal to the semiconductor device andproduce the electrical characteristic.
 8. The assembly of claim 7,wherein the signal generator is connected to the socket when the socketis placed in the micromill.
 9. The assembly of claim 1 furthercomprising a signal generator configured to send a signal to thesemiconductor device and produce the electrical characteristic which isrelated to a second order effect of a power draw of the silicon device.10. The assembly of claim 9, wherein the signal generator is configuredto apply a clock or a power waveform to the semiconductor device. 11.The assembly of claim 9, wherein the signal generator is connected tothe semiconductor device by lead wires.
 12. The assembly of claim 1further comprising a printed circuit board wherein the semiconductordevice is soldered to the printed circuit board.
 13. The assembly ofclaim 1, wherein the sensor is an electrical signal recording device.14. The assembly of claim 1, wherein the sensor is a current or avoltage sensor connected to a digital to analog converter.
 15. Theassembly of claim 1, wherein the sensor is an RF sensor.
 16. Theassembly of claim 1, wherein the mill is a micromill including a supportfor the semiconductor device that moves in both X and Y directions of amilling surface, a feed motor which moves the support in a Z directionnormal to the milling surface and a sensor for determining an amount offorce applied to the semiconductor device.
 17. A method of milling asemiconductor device including a circuit while detecting an endpoint ofa milling process, said method comprising: measuring an electricalcharacteristic of the semiconductor device with a sensor during millingin a mill; detecting an amount of strain in the semiconductor devicefrom the electrical characteristic; and determining when to stop millingbased on the amount of strain.
 18. The method according to claim 17,wherein the electrical characteristic is an analog measurement of powerversus time and further comprising segmenting the analog measurement ofpower versus time into a vector.
 19. The method according to claim 18further comprising conducting a machine learning technique to the vectorto relate a power measurement to strain in the semiconductor device. 20.The method according to claim 19 further comprising determining athickness of the remaining silicon being milled based on the strain inthe semiconductor device and controlling the milling based on the powermeasurement to stop milling when the thickness of the silicon becomestoo thin and before the circuit is damaged and wherein the powermeasurement is a second order effect on a power draw of thesemiconductor device.